Novel silver complexing agents

ABSTRACT

There are described novel acyclic silver complexing agents which are useful as silver halide solvents in photographic products, processes and compositions. Also disclosed are photographic compositions, products and processes in which the complexing agents are utilized.

This is a division of application Ser. No. 080,440, filed Oct. 1, 1979.

BACKGROUND OF THE INVENTION

This application relates generally to photography and more particularlyto novel silver complexing compounds which are useful in photographicproducts, processes and compositions.

Photographic processing compositions capable of forming water-solublecomplex silver salts are known to be useful in many types of silverhalide photography. In conventional or "tray" development, it iscustomary to fix the developed silver halide emulsion by applying asolution of silver halide solvent, i.e., silver halide complexing agentwhich forms a water-soluble silver complex with the residual silverhalide. The water-soluble silver complex thus formed and excess silverhalide solvent are then removed from the developed and fixed emulsion bywashing with water.

Silver halide solvents also have been employed in monobaths where asingle processing composition containing a silver halide developingagent, in addition to the silver halide solvent, is utilized for bothdeveloping and fixing an exposed photosensitive silver halide layer.Silver halide solvents also have been employed in diffusion transferphotographic processes. Such processes are now well known in the art;see, for example, U.S. Pat. Nos. 2,543,181; 2,647,056; 2,983,606; etc.In processes of this type, an exposed silver halide emulsion is treatedwith a processing composition whereby the exposed silver halide emulsionis developed and an imagewise distribution of diffusible image-formingcomponents is formed in the unexposed and undeveloped portions of thesilver halide emulsion. This distribution of image-forming components istransferred by imbibition to an image-receiving stratum in superposedrelationship with the silver halide emulsion to provide the desiredtransfer image. In diffusion transfer processes where a silver transferimage is formed, processing is effected in the presence of a silverhalide solvent which forms a diffusible complex with the undevelopedsilver halide. The soluble silver complex thus formed diffuses to thesuperposed image-receiving layer where the transferred silver ions aredeposited as metallic silver to provide the silver transfer image. Inpreparing silver prints in this manner, the image-receiving elementpreferably includes a silver precipitating agent, for example, heavymetal sulfides and selenides as described in U.S. Pat. No. 2,698,237.

The present invention is concerned with novel silver halide solvents andtheir use in photographic products, processes and compositions.

SUMMARY OF THE INVENTION

It is therefore the object of the invention to provide novel silvercomplexing agents which are useful as silver halide solvents.

It is another object of the invention to provide novel acyclic silvercomplexing agents.

It is a further object to provide photographic products, processes andcompositions employing such silver halide solvents.

BRIEF SUMMARY OF THE INVENTION

These and other objects and advantages of the invention are accomplishedby providing novel silver complexing agents which are useful as silverhalide solvents in photographic products, processes and compositions andwhich are represented by the general formula: ##STR1## wherein m is 2 or3; n is 2 or 3; R₁ and R₂ may be the same or different and may be H,alkyl, hydroxyalkyl or aminoalkyl having two to six carbon atoms; and R₃and R₄ may be the same or different and may be alkyl, alkoxyalkyl,hydroxyalkyl or aminoalkyl having two to six carbon atoms.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Specific preferred silver complexing agents which are suitable for useaccording to the invention are represented by the following formulas:##STR2##

These preferred silver complexing agents are stable in an alkalineenvironment, have a melting point less than about 50° C. and the log ofthe stability constant (β) for a 1:1 complex of the complexing agentwith silver is at least about 10.5. By "stable in an alkalineenvironment" is meant that the silver complexing agent retains at least75% of its silver complexing ability after being in a 1 N sodiumhydroxide solution for twenty-four hours at room temperature. Thesepreferred complexing agents can be used in a diffusion transferphotographic method for making positive transparencies, without washing,which are substantially completely free of crystals. The method isdescribed and claimed in applicants' copending application Ser. No.080,349 filed on even date herewith.

Various of the novel compounds of the invention may be synthesized byreacting N,N'-dimethyl-N,N'-bis(2-mercaptoethyl)ethylenediamine (for itspreparation see J. Amer. Chem. Soc., 98 6951 (1976) with the appropriatematerial, for example, ethylene oxide in aqueous solution to makecompound A and methyl iodide-sodium hydride in tetrahydrofuran to formcompound B.

The acyclic ligands wherein R₁ and R₂ are alkoxyalkyl can be prepared byinitially reacting ethylenediamine with an alkoxyaldehyde to form aSchiff's base, reducing the Schiff's base to form the symmetricallysubstituted bis(alkoxyalkyl) ethylenediamine and reacting the latterwith ethylene sulfide to formN,N'-dialkoxyalkyl-N,N'-bis(2-mercaptoethyl)ethylenediamine. Alkylationreactions of the latter may be carried out with alkylhalides,alkoxyalkylhalides, ethylene oxide and ethylene imine to obtainderivatives wherein R₃ and R₄ are alkyl, alkoxyalkyl, hydroxyalkyl andaminoalkyl. The analogous compounds wherein R₁ and R₂ are hydroxyalkylcan be prepared by starting withN,N'-bis(2-hydroxyalkyl)ethylenediamines, converting the hydroxyl groupsto protect them such as by reacting with dihydropyran to formtetrahydroxypyranylether, followed by reaction with ethylene sulfide toform the desiredN,N'-disubstituted-N,N'bis(2-mercaptoethyl)ethylenediamine. Alkylationof the latter with alkylhalides, alkoxyalkylhalides, ethylene oxide andethylene imine may be employed to obtain derivatives wherein R₃ and R₄are alkyl, alkoxyalkyl, hydroxyalkyl and aminoalkyl groups. After theligands are isolated they are deblocked such as by acid hydrolysis. Theprimary amine aminoalkyl derivatives can be prepared by initiallyreacting ethylenediamine and a cyanoalkylaldehyde to form a Schiff'sbase, reducing the Schiff's base to form a symmetrically substitutedcyanoalkyl ethylenediamine, and reacting the latter with ethylenesulfide to form the correspondingN,N'-bis(2-mercaptoethyl)ethylenediamines. The latter can be alkylatedwith various alkylating agents as described above followed by reductionof the cyano group to form the desired aminoalkyl substituted ligands.The tertiary amine aminoalkyl derivatives can be prepared by initiallyreacting ethylenediamine with a dialkylaminoalkylaldehyde to give aSchiff's base, reducing the Schiff's base to form a symmetricallysubstituted dialkylaminoalkyl ethylenediamine, reacting the latter withethylene sulfide and reacting the product with alkylating agents to formthe desired ligand. The desired ligands can be separated from the crudereaction products by first treating a methanol solution of the crudeproduct with silver thiocyanate to form a 1:1 ligand-silver thiocyanatecomplex which preferentially crystallizes from solution while theimpurities remain in the filtrate. Recrystallization of the complexfollowed by the precipitation of silver ions as silver sulfide withhydrogen sulfide and liberation of the free ligand by passing an aqueoussolution of the resulting thiocyanic acid complex through an anionexchange column provides essentially pure samples of the ligands.Alternatively, purification can be effected by chromatography of thecrude product mixture on silica gel, a more time-consuming procedure.

As mentioned previously, the compounds of the invention as representedby the general formula are useful as silver complexing agents inphotography. The log of the stability constant (β) for a 1:1 complex ofcompound A is 10.91±0.01 and that for compound B is 10.97±0.01. Thestability constants were determined by potentiometry, i.e., by titratingthe ligand with a standardized solution of silver perchlorate in mildlyalkaline, constant pH, constant ionic strength medium (0.05 M NaOH, 0.10M NaClO₄). All solutions and titrants were prepared carbonate-free andwith an ionic strength of 0.1 (NaClO₄). An argon atmosphere was usedthroughout. The indicating electrode was a silver specific ion type usedin conjunction with a sleeve-type double-function Ag/Ag Cl referenceelectrode.

In formulating photographic processing compositions utilizing theabove-described compounds, the compounds may be used singly or inadmixture with each other or with other silver halide solvents. Thetotal amount employed may vary widely depending upon the particularphotographic system and should be used, for example, in a quantitysufficient for fixing a developed negative in conventional "tray"processing or in a quantity sufficient to give a satisfactory transferprint in diffusion transfer processes under the particular processingconditions employed.

Though the silver halide solvents of the present invention are broadlyuseful in a variety of photographic processes of the type in whichwater-soluble silver complexes are formed from the unreduced silverhalide of a photoexposed and at least partially developed silver halidestratum, they find particular utility in diffusion transfer processes. Acomposition embodying the present invention specifically suitable foruse in the production of transfer images comprises, in addition to thesilver complexing agents of the above-described type, a suitable silverhalide developing agent. Examples of developing agents that may beemployed include hydroquinone and substituted hydroquinones, such as,tertiary butyl hydroquinone, 2,5-dimethyl hydroquinone,methoxyhydroquinone, ethoxyhydroquinone, chlorohydroquinone; pyrogalloland catechols, such as, catechol, 4-phenyl catechol and tertiary butylcatechol; aminophenols, such as, 2,4,6-triamino-orthocresol;1,4-diaminobenzenes, such as, p-phenylenediamine, 1,2,4-triaminobenzeneand 4-amino-2-methyl-N,N-diethylaniline; ascorbic acid and itsderivatives, such as ascorbic acid, isoascorbic acid and5,6-isopropylidene ascorbic acid, and other enediols, such astetramethyl reductic acid; and hydroxylamines, such as,N,N-di-(2-ethoxyethyl)hydroxylamine andN,N-di-(2-methoxyethoxyethyl)hydroxylamine.

In diffusion transfer processes, the processing composition, if it is tobe applied to the emulsion by being spread thereon in a thin layer, alsousually includes a viscosity-imparting reagent. The processingcomposition may comprise, for example, one or more silver halidesolvents of the present invention, one or more conventional developingagents such as those enumerated above, an alkali such as sodiumhydroxide or potassium hydroxide and a viscosity-imparting reagent suchas a high molecular weight polymer, e.g., sodium carboxymethyl celluloseor hydroxyethyl cellulose.

In one such transfer process, the processing solution is applied in auniformly thin layer between the superposed surfaces of a photoexposedphotosensitive element and an image-receiving element, for example, byadvancing the elements between a pair of pressure-applying rollers. Theelements are maintained in superposed relation for a predeterminedperiod, preferably for a duration of 15 to 120 seconds, during whichexposed silver halide is reduced to silver and unreduced silver halideforms a water-soluble, complex salt which diffuses through the layer ofsolution to the image-receiving element, there to be reduced to anargental image. At the end of this period, the silver halide element isseparated from the image-receiving element. Materials useful in such atransfer process are well known in the art.

The photosensitive element may be any of those conventionally used insilver diffusion transfer processes and generally comprises a silverhalide emulsion carried on a base, e.g., glass, paper or plastic film.The silver halide may be a silver chloride, iodide, bromide,iodobromide, chlorobromide, etc. The binder for the halide, thoughusually gelatin, may be a suitable polymer such as polyvinyl alcohol,polyvinyl pyrrolidone and their copolymers.

The image-receiving element preferably includes certain materials, thepresence of which, during the transfer process has a desirable effect onthe amount and character of silver precipitated on the image-receivingelement. Materials of this type are known in the art.

Separating of the silver halide element from the image-receiving elementmay be controlled so that the layer of processing composition is removedfrom the image-receiving element or the layer of processing compositionis caused to remain in contact with the image-receiving element, e.g.,to provide it with a protective coating. Techniques which enable suchresults to be accomplished as desired are described in U.S. Pat. No.2,647,054. In general, the processing reagents are selected so thattraces remaining after the solidified processing layer has beenseparated from the silver image or which remain in said layer adhered asa protective coating on the silver image, as indicated above, arecolorless or pale, so as not to appreciably affect the appearance of theimage and to have little or no tendency to adversely react with thesilver image.

The silver halide solvents of the present invention also may be employedin diffusion transfer processes adapted to provide positive silvertransfer images which may be viewed as positive transparencies withoutbeing separated from the developed negative silver image including suchprocesses adapted for use in forming additive color projection positiveimages. Diffusion transfer processes of this type are known in the art.See, for example, U.S. Pat. Nos. 3,536,488, 3,615,428 and 3,894,871. Thesubject compounds also find utility as silver halide solvents indiffusion transfer processes utilizing the properties of the imagewisedistribution of silver ions in the soluble silver complex made availablein the undeveloped and partially developed areas of a silver halideemulsion to liberate a reagent, e.g., a dye in an imagewise fashion, asdescribed in U.S. Pat. No. 3,719,489.

As noted above, in diffusion transfer film units the negative componentcomprising at least one photosensitive layer and the positive componentcomprising an image-receiving layer may be in separate sheet-likeelements which are brought together during processing and thereaftereither retained together as the final print or separated following imageformation.

Rather than the photosensitive layer and the image-receiving layer beingin separate elements, they may be in the same element. In one such filmunit, the image-receiving layer is coated on a support and thephotosensitive layer is coated on the upper surface of theimage-receiving layer. The liquid processing composition is appliedbetween the combined negative-positive element and a second sheet-likeelement or spreading sheet which assists in spreading the liquidcomposition in a uniform layer adjacent the surface of thephotosensitive layer.

Still other film units are those where the negative and positivecomponents together may comprise a unitary structure wherein theimage-receiving layer carrying the transfer image is not separated fromthe developed photosensitive layer(s) after processing, but bothcomponents are retained together as a permanent laminate. Such filmunits include those for providing positive silver transfer images whichmay be viewed as positive color transparencies, such as, those describedin U.S. Pat. No. 3,894,871. Other integral film units also include thoseadapted for forming a transfer image, in color or in black and white,viewable by reflected rather than by transmitted light. In addition tothe aforementioned photosensitive layer(s) and image-receiving layer,such film units include means for providing a reflecting layer betweenthe image-receiving and photosensitive layer(s) in order to mask thedeveloped photosensitive layer(s) and to provide a white background forviewing the transfer image. This reflecting layer may comprise apreformed layer of a reflecting agent included in the film unit or thereflecting agent may be provided subsequent to photoexposure, forexample, by including the reflecting agent in the processingcomposition. In addition to these layers, the laminate usually includesdimensionally stable outer layers or supports, at least one of which istransparent so that the resulting transfer image may be viewed byreflection against the background provided by the light-reflectinglayer. Integral negative-positive film units wherein the photosensitiveand image-receiving layers are retained as a permanent laminate afterprocessing are described, for example, in U.S. Pat. Nos. 3,415,644;3,647,437; and 3,594,165.

It will be appreciated that in the formation of color transfer images, adye image-providing material such as the compounds of U.S. Pat. No.3,719,489 may be associated with the photosensitive silver halide layeror layers of the negative component.

The diffusion transfer film units described above are employed inconjunction with means, such as, a rupturable container containing therequisite processing composition and adapted upon application ofpressure of applying its contents to develop the imagewise exposed filmunit.

The invention will now be described further in detail with respect tospecific preferred embodiments by way of examples, it being understoodthat these are illustrative only and the invention is not intended to belimited to the materials, conditions, process parameters, etc., recitedtherein. All parts and percentages are be weight unless otherwiseindicated.

EXAMPLES Preparation ofN,N'-dimethyl-N,N'-bis(2-mercaptoethyl)ethylenediamine

A 250 ml three-neck flask, equipped with a magnetic stirrer, additionfunnel, thermometer, and argon inlet was charged with a solution of 18.3g (0.208 mol) of N,N'-methylethylenediamine in 70 ml of benzene. Asolution of 25.1 g (0.417 mol) of ethylene sulfide in 10 ml of benzenewas added with stirring over a two-hour period to the solution in theflask under argon while maintaining the temperature of the solution inthe flask at 50°-55° C. The resulting clear solution was allowed toremain overnight at ambient temperature, washed with two 5 ml portionsof water and dried over magnesium sulfate. The solvent was removed underreduced pressure and 39.9 g (97% yield) of a colorless oil wereobtained.

The material is susceptible to air oxidation and therefore furtherpurification was not carried out. The material can be stored up to aweek under argon in a freezer without any significant deterioration.Since the compound has an extremely unpleasant odor and can cause severeskin allergy, careful handling is necessary.

EXAMPLE I Preparation of Compound A

A 500 ml hydrogenation bottle placed in a 5° C. cooling bath was chargedwith 6.3 g (30 m mol) of freshly preparedN,N'-dimethyl-N,N'-bis(2-mercaptoethyl)ethylenediamine in 35 ml ofcarbonate-free water. To the resulting aqueous suspension there wereadded rapidly, with vigorous stirring, 3 ml (60 m mol) of ethyleneoxide. The bottle was stoppered tightly and the contents allowed to stirfor about 8 hours at 5° C. and then for another 8 hours at 25° C. The ¹³C NMR spectrum of the resulting aqueous solution showed six major linesexpected for the desired ligand (compound A) accompanied by about 15-20%of unidentified impurities. Removal of the solvent under reducedpressure gave a colorless syrup which was dissolved in 75 ml ofdichloromethane, washed with two 5 ml portions of water, dried overmagnesium sulfate and concentrated to provide 6.5 g of a syrupy product.This product solidified upon being stored overnight in a refrigerator.Thin-layer chromatography on silica gel (in methanol) showed one majorspot accompanied by traces of more polar impurities.

For further purification a 5.5 g sample of the crude material wasstirred with 100 ml of ether until a fine, uniform suspension wasformed. The suspension was kept at 0° C. for about 2 hours in an icebath, the solid was filtered, washed with four 10 ml portions of etherand dried in vacuo to give 3.7 g of the ligand (compound A)--a colorlesssolid m.p. 43°-44° C. NMR spectra were consistent with the assignedstructure. The mass spectrum showed a weak m/e at 296 for the parent ionwith a strong P+1 at 297.

To assure the removal of any last traces of impurities, a small sampleof the ligand was chromatographed on silica gel using ethylacetate-methanol mixture or dichloromethane-methanol mixture as theelutant. After chromatographic separation the product was crystallizedfrom ethyl acetate-petroleum ether mixture at -15° C. to furnish pureligand as colorless needle-like crystals, m.p. 44°-45° C. C₁₂ H₂₈ N₂ S₂O₂ requires 48.61% C, 9.52% H, 9.45% N, 21.63% S and 10.79% O. Elementalanalysis gave 48.75% C, 9.60% H, 9.20% N and 21.53% S. NMR and massspectral data were consistent with the desired ligand.

EXAMPLE II Preparation of Compound B

To a 250 ml, three-neck flask equipped with a stirring bar, additionfunnel, argon inlet and a rubber septum were added 3.98 g of 50% sodiumhydride-oil dispersion. Most of the oil was removed by treating thedispersion with three 20-ml portions of petroleum ether under argon. 50ml of dry tetrahydrofuran (99.9%) were then introduced into thedispersion from a syringe followed by a dropwise addition, withstirring, of 8.5 g (40 m mol) ofN,N'-dimethyl-N,N'-bis(2-mercaptoethyl)ethylenediamine at a rate slowenough to keep frothing under control. The contents of the flask wereallowed to stir for an additional 15 minutes and then treated with asolution of 10.96 g (80 m mol) methyl iodide in 7 ml tetrahydrofuran,added dropwise with vigorous stirring over a period of about 30 minutes.Throughout the addition the temperature was maintained below 30° C. byoccasional cooling in an ice bath. The resulting reaction mixture wasallowed to stir at ambient temperature for 14 hours. Excess sodiumhydride was then destroyed with 5 ml of cold water and the solventremoved under reduced pressure.

The residue was taken up in 50 ml of ether, stirred for about 5 minutesand filtered to remove the suspended solids which were then washed withthree 10 ml portions of ether. The combined ether filtrate and washingswere washed with three 5 ml portions of water to remove any inorganicsalts and dried over magnesium sulfate. Removal of solvent under reducedpressure gave 7.9 g of an essentially colorless thin liquid. Thin-layerchromatography on silica gel with methanol showed one major spotaccompanied by some minor polar impurities.

To a solution of 7.5 g of the product in 50 ml of methanol were added5.1 g of silver thiocyanate in small portions with vigorous stirring.Initially, the silver salt appeared to go into solution quite rapidly;but toward the end of the addition, the rate of dissolution slowedconsiderably. The mixture was allowed to stir for 20 minutes afteraddition of the silver thiocyanate, diluted with 100 ml of methanol andfiltered through Celite 542 (a diatomaceous earth filter materialavailable from Johns-Manville) to remove insoluble material. Thefiltrate was concentrated under reduced pressure to about 30 ml andstored overnight in a refrigerator. Crystals developed and werecollected and recrystallized twice from methanol to provide 4.7 g ofnearly colorless solid m.p. 106°-107° C. C₁₁ H₂₄ N₃ S₃ Ag requires32.83% C, 6.01% H, 10.44% N, 23.90% S and 26.81% Ag. Elemental analysisgave 32.88% C, 5.89% H, 10.42% N, 23.72% S and 26.83% Ag. A ¹³ C NMRspectrum was consistent with the compound B--silver thiocyanate complex.

4.5 g of the complex were then dissolved in 50 ml of 60:40 (vol/vol)dichloromethane-ether mixture. The solution was cooled in an ice bathand treated with hydrogen sulfide gas to precipitate silver as silversulfide. Following complete precipitation stirring was carried out for15 minutes. The contents were filtered through Celite 542 and thefiltrate was concentrated under reduced pressure to give a clear liquid.The liquid was treated with a stoichiometric amount of aqueoustetramethylammonium hydroxide and the product was extracted withdichloromethane. The extract was washed with water and dried overmagnesium sulfate. The solvent was removed under reduced pressure, andthere were obtained 2.45 g of the ligand (compound B) as a clear, thinliquid. NMR spectra were consistent with the desired ligand.

EXAMPLE III

A film unit was prepared as follows: the light-sensitive elementcomprised a transparent polyester film base carrying on one surface anadditive color screen of approximately 1000 triplets per inch of red,blue and green filter screen elements in repetitive side by siderelationship; an approximately 4 micron thick polyvinylidene chloridebarrier layer; a nucleating layer comprising 0.23 mg/ft² of palladiumnuclei (as metal), 0.29 mg/ft² of gelatin, 0.35 mg/ft² of tin (as metal)and 0.47 mg/ft² of total chloride (associated with Pd and Sn); aninterlayer of 2.21 mgs/ft² of deacetylated chitin, 0.645 mg/ft² ofcopper acetate (dihydrate), 0.178 mg/ft² of sodium acetate and 0.194mg/ft² of alkyl phenoxy polyoxy ethylene glycol; a hardened gelatinosilver iodobromo emulsion coated at a coverage of about 90 mgs/ft² ofsilver, 120 mgs/ft² of gelatin, 53 mgs/ft² of Dow-620 carboxylatedstyrene butadiene latex and 4.83 mgs/ft² of dioctyl ester of sodiumsuccinic acid (a surfactant); and an antihalo topcoat of 300 mgs/ft² ofgelatin, 175 mgs/ft² of Dow-620 carboxylated styrene butadiene latex,0.3 mg/ft² of dioctyl ester of sodium succinic acid, 5.2 mg/ft² ofpyridinium-bis-1,5(1,3-diethyl-2-thiol-5-barbituric acid)pentamethineoxanol, 7.0 mgs/ft² of 4-(2-chloro-4-di-methylaminobenzaldehyde)-1-(p-phenylcarboxylic acid)-3-methyl pyrazolone-5 and 4.9mgs/ft² of benzimidazole-2-thiol gold Au⁺¹ complex (as gold).

The cover sheet comprised a 4 mil thick polyethylene terephthalatephotographic film support having a thin coating on one surface toprepare the support for coating. Coated on the support in the followingorder were:

1. An acid providing layer formed by combining 60 parts by volume of a30% solution of the half butyl ester of ethylene maleic anhydride inmethyl ethyl ketone and 40 parts by volume of a solution of 5.7% ButvarB-72 (available from Monsanto), 63.3% ethyl acetate and 31% n-butanoland coating the mixture on the support at a dry coverage of about 2.45gms/ft² ; and

2. A gelatin layer formed by coating a water solution containing 10%deionized gelatin and 0.05% Emulphor ON-870 (available from AntaraChemical Co.) to provide a dry coverage of about 1 gm/ft².

Film units as described above were exposed through the additive colorscreen to a step wedge and processed by spreading a layer of processingcomposition less than about 3 mils thick between the light sensitiveelement and the cover sheet. The processing compositions were preparedby adding 0.08725 g of compound A and 0.05405 of compound B,respectively, to 10 ml of the following formulation:

    ______________________________________                                        Water                   82.36 g                                               Sodium hydroxide        7.265 g                                               Hydroxyethyl cellulose  2.811 g                                               Sodium sulfite          2.54 g                                                Tetramethyl reductic acid                                                                             3.17 g                                                Dodecyl-N,N-dipyridinium                                                      dibromide               1.78 g                                                4-aminopyrazolo(3,4-d)-                                                       pyrimidine              0.016 g                                               5-bromo-6-methyl-4-                                                           azabenzimidazole        0.016 g                                               Thiazolidine-2-thione   0.035 g                                               ______________________________________                                    

After an imbibition period of about one minute the maximum and minimumdensities for the resultant images were measured on a transmissiondensitometer. The results were as follows:

    ______________________________________                                        Compound             Red      Green   Blue                                    ______________________________________                                                   D.sub.max 1.97     1.97    2.07                                               D.sub.min 0.54     0.62    0.76                                               D.sub.max 1.49     1.51    1.68                                    B                                                                                        D.sub.min 0.48     0.57    0.69                                    ______________________________________                                    

The resultant images did not have any apparent crystals when inspectedvisually ten days after processing. The images were stored under ambientconditions during the interim.

It will be apparent that the relative proportions of the subject silverhalide solvents and of the other ingredients of the processingcompositions may be varied to suit the requirements of a givenphotographic system. Also, it is within the scope of this invention tomodify the formulations set forth above by the substitution of alkalies,antifoggants and so forth other than those specifically mentioned. Wheredesirable, it is also contemplated to include in the processingcompositions, other components as commonly used in the photographic art.

Although the invention has been described with respect to variouspreferred embodiments thereof, it is not intended to be limited theretobut rather those skilled in the art will recognize that modificationsand variations may be made therein which are within the spirit of theinvention and the scope of the claim.

What is claimed is:
 1. A compound represented by the formula: ##STR3##wherein m is 2 or 3; n is 2 or 3; R₁ and R₂ may be the same or differentand may be H, alkyl, alkoxyalkyl, hydroxyalkyl or aminoalkyl having twoto six carbon atoms; and R₃ and R₄ may be the same or different and maybe alkyl, alkoxyalkyl or aminoalkyl having two to six carbon atoms.
 2. Acompound as defined in claim 1 which is represented by the formula:##STR4##
 3. A compound as defined in claim 1 which is represented by theformula: ##STR5##